This invention relates to the field of CMOS integrated circuits and, more particularly, to CMOS comparators.
A comparator is a device which is generally used to compare two voltage inputs and switch its output to indicate which of the inputs is larger. Typically comparators include at least two analog voltage inputs, which are compared to each other to determine the appropriate digital output. If the difference between the input voltages is positive then a comparator outputs a high value, and if the difference is negative then a comparator outputs a low value. Because of this behavior, comparators are popular for triggering events in digital logic based on the relative values of analog signals. For example, comparators may be used to initiate or terminate an operation depending on the difference between two particular signals.
Comparators have several performance parameters which determine their usefulness for various functions. Comparator gain is the minimum difference between the two input voltages which is required for the comparator to switch its output. A higher comparator gain corresponds to a more precise comparator. Common-mode voltage range is the range of input voltages over which a comparator functions correctly. Common-mode voltage refers to the average voltage of the input signals. A wider common-mode voltage range indicates a comparator which may interface with a wider range of input voltages. Speed corresponds to how fast a comparator's outputs respond to input-voltage changes. A higher speed indicates a quicker response time. Robustness determines how sensitive a comparator is to environmental (e.g., processing, temperature, supply voltage) variations. A more robust comparator corresponds to a less sensitive comparator. Range of output swing determines the difference between the high output value and the low output value. A larger output swing indicates a greater difference between the voltage levels of high and low outputs.
A comparator with a high output swing is traditionally preferable because it easily interfaces with logic circuits requiring large input swings. For example, the term “rail-to-rail output swing” is commonly used to denote an output swing which goes from ground to the supply voltage. These large output swings may be important to ensure the proper functioning of the subsequent digital logic.
Typically, comparator design seeks to maximize each of these different parameters to create a comparator with all around solid performance. However, in some instances it may be desirable to sacrifice certain parameters for higher performance in others. For example, in some comparator applications a higher speed may be valued over a larger output swing.
It would therefore be desirable to design a comparator with a high-speed reduced-output-swing. It would also be desirable for this high-speed reduced-output swing comparator to have a self-biased configuration, a fully-complementary design, and a rail-to-rail input common-mode range.